Molecular ecology of nitrogen fixation
in the sea: From gene expression
to global carbon cycling

Jonathan P. Zehr, Ph.D.
University of California, Santa Cruz

Wednesday, September 5, 2001
3:00 p.m.–Pacific Forum

The availability of
nitrogen is one of the crucial factors constraining oceanic primary
productivity and, ultimately, controlling global atmosphere-ocean carbon
balances. Nitrogen-fixing organisms should be keystone microorganisms in
nitrogen-limited waters, and yet only a few nitrogen-fixing microorganisms
have been previously reported from oceanic waters.

Nitrogenase genes and nitrogenase gene transcripts (mRNA), indicative
of active transcription of the nitrogenase genes, were detected from
surface waters at station ALOHA in the North Pacific Subtropical Gyre.
Nitrogenase genes were expressed in
a distinct daily pattern throughout the upper 150 meters of the water
column. Analysis of the gene sequence led to identification and
cultivation of a novel unicellular cyanobacterium, which appears to be an
important, previously unrecognized nitrogen fixer particularly in the
North Pacific Subtropical Gyre. These results have important implications
for the global controls on primary productivity by the availability of
nitrogen.